This invention relates to a bit generator for determining a secret encryption key and a corresponding process. Its applications include transmission of secret information. More particularly, it can replace quantum cryptology.
Mathematicians have always searched for protected communication techniques for transmitting secret information. The principle based on the specific features of quantum mechanics to generate random encoding keys was proposed in the 1970s by Stephen Wiesner [1]. Charles E. Bennett et al contributed to demonstrating the feasibility of the concept [2].
Quantum cryptology is based on the fact that the quantum mechanics measurement is a random and irreversible phenomenon that irrecoverably disturbs the measured object. Thus, interception of the message by a third party will inevitably modify the quantum state of the object so that illegal eavesdropping can be detected, or conversely it is possible to be assured that the message has not been intercepted. In this case, keys that could be used to encode messages later can be transmitted being certain that the key is unique and has been actually kept secret.
The principle used to transmit a decryption key is then as follows.
An operator A (conventionally called Alice in the cryptology specialists language) sends a series of particles with a certain spin, or photons with a certain polarization, to a destination B (conventionally called Bob). Alice has two orthogonal spin measurement reference systems that are denoted a and b. When measured according to reference system a, the spin of a particle is +1 or xe2x88x921 (in a suitable system of units). If the spin of the same particle is measured for a second time in the same reference system, in this case a, obviously the same value will be found. But if the said spin is measured in the other reference system b, then a random value +1 or xe2x88x921 will be found independently of the value found in the first measurement.
In a series of successive operations, Alice chooses a series of reference systems a or b at_random. She measures the spin of a new particle in each reference system and sends it to Bob. She notes her choices and the results of her measurements. In turn, Bob chooses a series of reference systems a or b at random, and uses these reference systems to successively measure the spins of the particles that Alice sends to him. He also notes his choices and his results. Obviously, if Bob chose the same reference system as Alice (by chance) which on average would happen in half of all cases, he would find the same result as she did. However if he chose the other reference system, he would obtain a random result compared with the spin measured by Alice. Therefore, if Alice and Bob exchange their choices of reference systems a or b for each particle in a second step, they deduce which were the common choices and which were the series of the +1 or xe2x88x921 measured spins that they have in common. This series of common spins forms the secret key that they can share.
For security reasons, they can inform each other of part of this key; if this part is actually common, then the message has not been intercepted. Otherwise, they must begin the operation again until they obtain a secret and tested common key.
This technique for determining an encryption key is difficult to use since it requires lasers that emit photons individually, with a particular polarization. The purpose of this invention is to overcome this disadvantage.
Consequently, the invention proposes a device called a bit generator which simulates quantum means according to prior art in a specific manner but uses means that are much easier to implement, such as binary data memories and calculation circuits. This device then no longer works with spins or quantum states, but rather with bits and memory contents which is much more convenient.
According to the invention, one or more memory cells may be used, for example with two bits, with a first bit called the reference system bit, the value of which is denoted a or b (in the case of an environment with two reference systems) and a second bit called the measurement bit denoted +1 or xe2x88x921. This type of cell may be in one of four logical states, namely (a, +1) or (a, xe2x88x921) or (b, +1) or (b, xe2x88x921). The initial state is generated randomly inside the circuit defined by the invention. In order to measure the state of a cell, a reference system bit is applied to it, in other words a or b, and a logical transition of the initial state to a final state is provoked, which is a random type of transition with a certain probability depending on the initial state and the final state and the applied reference system bit. The final state reflects the result of the measurement in the applied reference system.
Therefore, this type of generator has all the attributes of quantum objects and it can be used to determine secret encryption keys.
Naturally, the invention is not restricted to a single cell, or to two bits per cell, but covers all cases of n cells with k bits, where n and k are arbitrary integers.
More precisely, the purpose of this invention is a bit generator for determining a secret encryption key, characterized in that it is composed of an electronic circuit comprising:
an input to which an input signal can be applied comprising a group of n bits, called reference system bits, where n is an integer equal to at least 1,
an output on which an output signal can be produced composed of a group of n bits, called the measurement bits,
a memory with n memorization cells each memorizing k bits, where k is an integer equal to at least 2, with reference system bits and measurement bits, each cell thus being in a logical state defined by its k bits, this state not being readable from outside the electronic circuit, this memory being provided with an output connected to the output from the generator and outputting measurement bits from the n cells,
a calculation means connected to the memory and with one control input connected to the generator input and into which reference system bits are input, this calculation means being capable of provoking a random transition for each cell between the state of the cell called the initial state read on reception of the reference system bit corresponding to the cell, and another state called the final state, the probability of this transition depending on the initial state, the received reference system bit and pre-determined probability equations are defined in the calculation means.
Another purpose of this invention is a process for determining a secret encryption key common to a first user and a second user, this process using the generator described above. This process is characterized in that:
the first user applies a first group of random input bits to the generator input, and collects a first group of output bits at the output from the generator, and then sends the generator to the second user,
the second user applies a second group of random input bits to the input of this generator and collects a second group of output bits at the output from the generator,
the first user transmits the first group of input bits in plain text to the second user, and the second user transmits the second group of input bits that he used to the first user, in plain text,
the first and the second users identify the input bits common to the first and the second group of input bits and determine the bits corresponding to the common input bits in the first and in the second group of output bits,
the first and second users use at least part of the output bits corresponding to the common input bits as the common secret encryption key.
Preferably, the first and second users check the output bits corresponding to the common input bits to make sure that the bits in the first group and in the second group are actually identical, and use the remaining bits to build up their common secret key